In order to achieve high component efficiencies for a turbine and ultimately a gas turbine cycle itself small tip clearances are required. This is even more important for small gas turbines where the relative clearance is increased due to manufacturing limitations and tolerances. One way of controlling the tip clearance in the turbine is to actively cool the casing. For example, the thermal expansion of the casing is matched with the rotor to minimize the clearance without taking the risk of interference during transient and hot restarts.
This type of cooling requires cooling air being bled from the compressor. In small gas turbines having a lower overall pressure ratio and fewer bleed points in the compressor the available air supply is often of a much higher pressure and temperature than needed to perform the cooling task on the turbine casing. This may have at least two effects, one being that due to the higher temperature of the cooling air a minimum clearance can not be achieved at all or only by using excessive mass flows, the other being that after the cooling task the air is released in the flue gases still having a surplus pressure without producing any work which generates additional performance losses.
The first category of solutions is the use of a higher pressure and temperature of the cooling air than required. Another alternative and improvement of the first category is to pre-cool the air before it is applied to the turbine casing. This can be done in a heat exchanger, for example an air-air heat exchanger, an air-water heat exchanger or an air-fuel heat exchanger, or by injecting evaporating water into the air stream. The pre- cooling requires more components and/or more systems, adds costs and potentially decreases the reliability and availability.
A different approach is to use a low expansion material in the turbine casing which allows the turbine rotor to perform the vast portion of the relative movement between the blade tip and the casing minimizing the clearance during operation. This solution does not require any extra cooling air, is able to deal with transient movements but is slow and requires expensive materials in large components, which means that it adds costs.
In U.S. Pat. No. 5,611,197 a closed circuit air cooled turbine where air is bled from the compressor and is used to cool the turbine casing is described. After cooling the turbine casing the air is passed through a heat exchanger and is then injected in the compressor.
In EP 1 013 937 B1 a compressor bleed point at the tip region of the compressor blade row is disclosed.
In U.S. Pat. No. 6,422,807 B1 a closed circuit cooling of a turbine casing where the cooling medium is circulated through internal cavities and where the accumulated heat is removed in heat exchangers is described.
In U.S. Pat. No. 4,329,114 an active clearance control system for a compressor based on convective flow is disclosed.
In U.S. Pat. No. 6,412,270 B1 a method of mixing two bleed streams for a compressor using an ejector before using air for cooling or sealing purposes in a turbine is described.
In U.S. Pat. No. 4,711,084 the use of an ejector in a compressor low pressure air bleed conduit to increase the pressure is shown.
In U.S. Pat. No. 4,645,415 a cooling system where the cooling air is cooled by the flow through a secondary air path of a turbofan gas turbine is described.